The invention relates to a quick test of the operability of a subscriber circuit of a digital telephone exchange.
A test procedure for testing the operability is already known in which the subscriber circuit is disconnected from the subscriber line through the use of at least one relay, and is connected to a central test unit.
This procedure has disadvantages since additional space-consuming and costly relays must be located on the subscriber line module, on which there may be a number of subscriber circuits. Since the operability test is carried out using a central, external test unit which occupies the space of two subscriber line modules, it furthermore means that at least 32 fewer subscribers can be connected to the telephone exchange.
In addition, the test procedure can be applied by the central test unit to individual subscriber circuits only serially.
Furthermore, Published European Patent Application EP 0 451 759 discloses how a test of the transmission characteristics of a subscriber circuit can be integrated together with the connected subscriber lines in the subscriber circuit. This principle is based on accessing various points in the reception path and in the transmission path of a signal processor contained in the subscriber circuit in order to use the voltages which occur at these points and are either caused or are fed in separately by the normal telephone operation to determine specific transmission characteristics of a subscriber circuit together with subscriber lines, by forming a paired correlation product. However, this procedure is not suitable for quickly testing the operability of a subscriber circuit, since the signal processor is severely burdened with storage and computation tasks as a result of the numerous voltage values to be evaluated and as a result of the complex calculation method for the transmission characteristics, such as the return loss or transmission level, for which reason it takes too long to determine a final result for a quick test procedure.
It is accordingly an object of the invention to provide a method for testing an operability of a subscriber circuit of a digital telephone exchange which overcomes the above-mentioned disadvantages of the heretofore-known methods of this general type and which is as quick as possible while, at the same time, the circuitry complexity and costs are kept low.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for quickly testing an operability of a subscriber circuit of a digital telephone exchange, which includes the steps of:
producing, with a tone generator provided in a subscriber circuit, an alternating voltage having a given frequency;
amplifying the alternating voltage with an amplifier provided in the subscriber circuit for supplying an amplified alternating voltage;
feeding the amplified alternating voltage into connecting paths of the subscriber circuit, the connecting paths being provided for a connection to respective lines of a subscriber line;
diverting the amplified alternating voltage to ground via respective capacitors connected to the connecting paths for the respective lines;
measuring respective alternating current levels in the connecting paths for providing respective measured alternating current levels;
forming a sum of the respective measured alternating current levels for providing a summed alternating current level;
comparing, in a frequency-selective manner, the summed alternating current level with a current threshold value; and
selectively outputting a positive test result, if the summed alternating current level exceeds the current threshold value, and confirming a defect, if the summed alternating current level is below the current threshold value.
The principle on which the invention is based is, in particular, to integrate the test functions of the test procedure in the subscriber circuit using already existing components, without significantly burdening the signal processor by time-consuming storage and computation tasks.
This is achieved in that a tone generator which is located in the subscriber circuit and whose task is primarily to transmit a search-tone signal when a defect occurs, in order to make it easier for the maintenance personnel to trace the defect, produces an alternating voltage. The alternating voltage is fed, using an amplifier (contained in the subscriber circuit) for the alternating current that is produced, into the connecting paths which are used to connect the lines of a subscriber line. The alternating currents, which are caused by the alternating voltage and are diverted to ground via capacitors connected to the connecting paths in order to ensure electromagnetic compatibility, are measured. The measured alternating-current levels are then added and are compared on a frequency-selective basis with a current threshold value. A defect in the subscriber circuit is confirmed if the total alternating-current level is below the current threshold value.
Thus, using the test procedure according to the invention, it is possible to eliminate relays on the subscriber line module on which the subscriber circuit is located.
A further advantage is achieved by the procedure according to the invention in that the central external test unit becomes superfluous, and two module plug-in slots are thus available for further subscriber connections (approx. 32 subscribers).
In addition, the test procedure for operability of a subscriber circuit can be carried out in a decentralized manner, and in parallel for the individual subscriber circuits.
Also, the signal processor is not additionally burdened by the test procedure according to the invention with a time-consuming determination procedure for transmission characteristics, but is burdened only with a process of comparing current levels, so that the test procedure can be carried out quickly in the manner according to the invention.
According to a preferred mode of the invention, the test procedure is advantageously expanded by repeating the procedure specified above if the test result is negative, with the alternating voltage produced by the tone generator being at a different frequency when the test is repeated. The presence of a defect in the subscriber circuit is confirmed only if the level is once again below the current threshold value.
In accordance with another mode of the invention, the magnitude of the current threshold value is made dependent on the frequency of the alternating voltage produced by the tone generator, taking into account the frequency dependency of the impedance of the capacitors.
The test procedure according to the invention is preferably supplemented such that the test procedure is immediately terminated if a subscriber who is connected to the subscriber circuit to be tested lifts the handset of a telephone or of some other terminal. Otherwise the subscriber could hear a test tone in the telephone earpiece in addition to a dialing tone.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for quickly testing the operability of a subscriber circuit, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.